This specification describes a privacy-preserving, space-efficient, and high-performance mechanism for publishing status information such as suspension or revocation of Verifiable Credentials.

This document is experimental and is undergoing heavy development. It is inadvisable to implement the specification in its current form. An experimental implementation is available.

Introduction

It is often useful for an issuer of verifiable credentials [[VC-DATA-MODEL]] to link to a location where a verifier can check to see if a credential has been suspended or revoked. There are a variety of privacy and performance considerations that are made when designing, publishing, and processing status lists.

One such privacy consideration happens when there is a one-to-one mapping between a verifiable credential and a URL where the status is published. This type of mapping enables the website that publishes the URL to correlate the holder, time, and verifier when the status is checked. This could enable the issuer to discover the type of interaction the holder is having with the verifier, such as providing an age verification credential when entering a bar. Being tracked by the issuer of a driver's license when entering an establishment violates a privacy expectation that many people have today.

Similarly, there are performance considerations that are explored when designing status lists. One such consideration is where the list is published and the burden it places from a bandwidth and processing perspective, both on the server and the client fetching the information. In order to meet privacy expectations, it is useful to bundle the status of large sets of credentials into a single list to help with herd privacy. However, doing so can place an impossible burden on both the server and client if the status information is as much as a few hundred bytes in size per credential across a population of hundreds of millions of holders.

The rest of this document proposes a highly compressible, bitstring-based status list mechanism with strong privacy-preserving characteristics, that is compatible with the architecture of the Web, is highly space-efficient, and lends itself well to content distribution networks. As an example of using this specification to achieve a number of beneficial privacy and performance goals, it is possible to create a status list that can be constructed for 100,000 verifiable credentials that is roughly 12,500 bytes in size in the worst case. In a case where a few hundred credentials have been revoked, the size of the list is less than a few hundred bytes while providing privacy in a herd of 100,000 individuals.

Conceptual Framework

This section outlines the core concept utilized by the status list mechanism described in this document. At the most basic level, status information for all verifiable credentials issued by an issuer are expressed as simple binary values. The issuer keeps a bitstring list of all verifiable credentials it has issued. Each verifiable credential is associated with a position in the list. If the binary value of the position in the list is 1 (one), the verifiable credential is revoked, if it is 0 (zero) it is not revoked.

One of the benefits of using a bitstring is that it is a highly compressible data format since, in the average case, large numbers of credentials will remain unrevoked. This will ensure long sections of bits that are the same value and thus highly compressible using run-length compression techniques such as GZIP [[RFC1952]]. The default bitstring size is 16KB (131,072 entries), and when only a handful of verifiable credentials are revoked, the compressed bitstring size is reduced down to a few hundred bytes.

Another benefit of using a bitstring is that it enables large numbers of verifiable credential statuses to be placed in the same list. This specification utilizes a minimum bitstring length of 131,072 (16KB). This population size ensures an adequate amount of herd privacy in the average case. If better herd privacy is required, the bitstring can be made to be larger.

diagram showing a list of boxes at the top of the image with two of them in red depicting revoked credentials. Text beside the boxes to the right reads 16 kilobytes. An depiction of the boxes being GZIP compressed into a cylinder on the bottom of the page shows that compression has resulted in a final size of 135 bytes.
A visual depiction of the concepts outlined in this section.

Terminology

Data Model

The following sections outlines the data model for this document.

StatusList2021Entry

When an issuer desires to enable status information for a verifiable credential, they MAY add a credentialStatus property that uses the data model described in this specification.

Property Description
id The constraints on the id property are listed in the Verifiable Credentials Data Model specification [[VC-DATA-MODEL]]. The value is expected to be a URL that identifies the status information associated with the verifiable credential. It MUST NOT be the URL for the status list.
type The type property MUST be StatusList2021Entry.
statusPurpose The purpose of the status entry MUST be a string. While the value of the string is arbitrary, the following values MUST be used for their intended purpose:
Value Description
revocation Used to cancel the validity of a verifiable credential. This status is not reversible.
suspension Used to temporarily prevent the acceptance of a verifiable credential. This status is reversible.
statusListIndex The statusListIndex property MUST be an arbitrary size integer greater than or equal to 0, expressed as a string. The value identifies the bit position of the status of the verifiable credential.
statusListCredential The statusListCredential property MUST be a URL to a verifiable credential. When the URL is dereferenced, the resulting verifiable credential MUST have type property that includes the StatusList2021Credential value. 
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3id.org/vc/status-list/2021/v1"
  ],
  "id": "https://example.com/credentials/23894672394",
  "type": ["VerifiableCredential"],
  "issuer": "did:example:12345",
  "issued": "2021-04-05T14:27:42Z",
  "credentialStatus": {
    "id": "https://example.com/credentials/status/3#94567"
    "type": "StatusList2021Entry",
    "statusPurpose": "revocation",
    "statusListIndex": "94567",
    "statusListCredential": "https://example.com/credentials/status/3"
  },
  "credentialSubject": {
    "id": "did:example:6789",
    "type": "Person"
  },
  "proof": { ... }
}

StatusList2021Credential

When a status list is published, the result is a verifiable credential that encapsulates the status list. The following section describes the format of the verifiable credential that encapsulates the status list:

Property Description
id The verifiable credential that contains the status list MAY express an id property that matches the value specified in statusListCredential for the corresponding StatusList2021Entry (see ).
type The verifiable credential that contains the status list MUST express a type property that includes the StatusList2021Credential value.
validFrom The earliest point in time at which the status list is valid. This property is defined in the Verifiable Credentials Data Model specification in Section 4.6: Validity Period.
validUntil The latest point in time at which the status list is valid. This property is defined in the Verifiable Credentials Data Model specification in Section 4.6: Validity Period. Implementations that consume the status list SHOULD cache the status list verifiable credential until this time. Implementations that publish the status list are expected to align any protocol-specific caching information, such as the HTTP `Cache-Control` header, with the value in this field.
credentialSubject.type The type of the credential subject, which is the status list, MUST be StatusList2021.
credentialSubject.statusPurpose The purpose of the status entry MUST be a string. While the value of the string is arbitrary, the following values MUST be used for their intended purpose:
Value Description
revocation Used to cancel the validity of a verifiable credential. This status is not reversible.
suspension Used to temporarily prevent the acceptance of a verifiable credential. This status is reversible.
credentialSubject.encodedList The encodedList property of the credential subject MUST be the GZIP-compressed [[RFC1952]], base-64 encoded [[RFC4648]] bitstring values for the associated range of verifiable credential status values. The uncompressed bitstring MUST be at least 16KB in size. The bitstring MUST be encoded such that the first index, with a value of zero (0), is located at the left-most bit in the bitstring and the last index, with a value of one less than the length of the bitstring (bitstring_length - 1), is located at the right-most bit in the bitstring. Further information on bitstring encoding can be found in Section . 
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3id.org/vc/status-list/2021/v1"
  ],
  "id": "https://example.com/credentials/status/3",
  "type": ["VerifiableCredential", "StatusList2021Credential"],
  "issuer": "did:example:12345",
  "issued": "2021-04-05T14:27:40Z",
  "credentialSubject": {
    "id": "https://example.com/status/3#list",
    "type": "StatusList2021",
    "statusPurpose": "revocation",
    "encodedList": "H4sIAAAAAAAAA-3BMQEAAADCoPVPbQwfoAAAAAAAAAAAAAAAAAAAAIC3AYbSVKsAQAAA"
  },
  "proof": { ... }
}

Algorithms

The following section outlines the algorithms that are used to generate and validate status lists as described by this document.

Generate Algorithm

The following process, or one generating the exact output, MUST be followed when producing a StatusList2021Credential:

  1. Let issued credentials be a list of all issued verifiable credentials.
  2. Let RLC be an unsigned StatusList2021Credential without the encodedList property set.
  3. Generate a compressed bitstring by passing issued credentials to the Bitstring Generation Algorithm.
  4. Set the encodedList to compressed bitstring.
  5. Generate a proof for the RLC and publish it to the endpoint listed in the verifiable credential.

Validate Algorithm

The following process, or one generating the exact output, MUST be followed when validating a verifiable credential that is contained in a StatusList2021Credential:

  1. Let credentialToValidate be a verifiable credentials containing a credentialStatus entry that is a StatusList2021Entry.
  2. Let status purpose be the value of statusPurpose in the credentialStatus entry in the credentialToValidate.
  3. Verify all proofs associated with the credentialToValidate. If a proof fails, return a validation error.
  4. Verify that the status purpose matches the statusPurpose value in the statusListCredential.
  5. Let compressed bitstring be the value of the encodedList property of the StatusList2021Credential.
  6. Let credentialIndex be the value of the statusListIndex property of the StatusList2021Entry.
  7. Generate a revocation bitstring by passing compressed bitstring to the Bitstring Expansion Algorithm.
  8. Let status be the value of the bit at position credentialIndex in the revocation bitstring.
  9. Return true if status is 1, false otherwise.

Bitstring Generation Algorithm

The following process, or one generating the exact output, MUST be followed when generating a status list bitstring. The algorithm takes a issuedCredentials list as input and returns a compressed bitstring as output.

  1. Let bitstring be a list of bits with a minimum size of 16KB, where each bit is initialized to 0 (zero).
  2. For each bit in bitstring, if there is a corresponding statusListIndex value in a revoked credential in issuedCredentials, set the bit to 1 (one), otherwise set the bit to 0 (zero).
  3. Generate a compressed bitstring by using the GZIP compression algorithm [[RFC1952]] on the bitstring and then base64-encoding [[RFC4648]] the result.
  4. Return the compressed bitstring.

Bitstring Expansion Algorithm

The following process, or one generating the exact output, MUST be followed when expanding a compressed status list bitstring. The algorithm takes a compressed bitstring as input and returns a uncompressed bitstring as output.

  1. Let compressed bitstring be a compressed status list bitstring.
  2. Generate an uncompressed bitstring by using the base64-decoding [[RFC4648]] algorithm on the compressed bitstring and then expanding the output using the GZIP decompression algorithm [[RFC1952]].
  3. Return the uncompressed bitstring.

Privacy Considerations

This section details the general privacy considerations and specific privacy implications of deploying this specification into production environments.

Revocation Bitstring Length

This document specifies a minimum revocation bitstring length of 131,072, or 16KB uncompressed. This is enough to give holders an adequate amount of herd privacy if the number of verifiable credentials issued is large enough. However, if the number of issued verifiable credentials is a small population, the ability to correlate an individual increases because the number of allocated slots in the bitstring is small. Correlating this information with, for example, where the geographic request came from can also help to correlate individuals that have received a credential from the same geographic region.

Verifier Caching

It is possible for verifiers to increase the privacy of the holder whose verifiable credential is being checked by caching status lists that have been fetched from remote servers. By caching the content locally, less correlatable information can be inferred from verifier-based access patterns on the status list.

Content Distribution Networks

The use of content distribution networks by issuers can increase the privacy of holders by reducing or eliminating requests for the status lists from the issuer. Often, a request for a revocation list will be served by an edge device and thus be faster and reduce the load on the server as well as cloaking verifiers and holders from issuers.

Malicious Issuers and Verifiers

In general, the herd privacy protections offered by this specification can be circumvented by malicious issuers and verifiers. Its privacy benefits can only be realized when issuers and verifiers intend to avoid tracking or sharing the presentation of particular credentials.

A malicious issuer might intentionally attack herd privacy by creating a unique status list per credential issued in order to establish a 1-1 mapping to track when a verifier processes a specific credential. Similarly, they could establish another a 1-1 mapping by using a different cryptographic key for every credential issued that is tracked in a status list.

A malicious verifier might intentionally attack herd privacy by sharing information from presented credentials with a malicious issuer.

Security Considerations

There are a number of security considerations that implementers should be aware of when processing data described by this specification. Ignoring or not understanding the implications of this section can result in security vulnerabilities.

While this section attempts to highlight a broad set of security considerations, it is not a complete list. Implementers are urged to seek the advice of security and cryptography professionals when implementing mission critical systems using the technology outlined in this specification.

Bitstring Encoding

It is critical that implementers pay particular attention to the way that they encode and decode bitstrings. Failure to do so can result in checking the wrong bitstring index for a given credential, leading to a misinterpretation of its present state (e.g., mistaking a revoked status for an unrevoked status). As stated in Section , bitstrings are encoded such that the first (zeroth) index refers to the left-most bit of the bitstring array. The diagram below demonstrates the proper layout for an uncompressed bitstring.

a diagram showing two wide rectangles shown side-by-side. Each rectangle is partitioned into eight boxes to represent the 8 bits in a byte. The left rectangle is labeled 'First byte' and the right rectangle is labeled 'Last byte'. The left rectangle's left-most bit box is pointed to by an arrow labeled 'index: 0'. The right rectangle's right-most bit box is pointed to by an arrow labeled 'index: length - 1'.
A visual depiction of the bitstring layout.

For example, if a bitstring is 131,072 bits in size (16KB), the first index will be 0, and the last index will be 131,071.

Accessibility Considerations

There are a number of accessibility considerations implementers should be aware of when processing data described in this specification. As with any web standards or protocols implementation, ignoring accessibility issues makes this information unusable to a large subset of the population. It is important to follow accessibility guidelines and standards, such as [[WCAG21]], to ensure all people, regardless of ability, can make use of this data. This is especially important when establishing systems utilizing cryptography, which have historically created problems for assistive technologies.

This section details the general accessibility considerations to take into account when utilizing this data model.

Write accessibility considerations.

Internationalization Considerations

There are a number of internationalization considerations implementers should be aware of when publishing data described in this specification. As with any web standards or protocols implementation, ignoring internationalization makes it difficult for data to be produced and consumed across a disparate set of languages and societies, which would limit the applicability of the specification and significantly diminish its value as a standard.

This section outlines general internationalization considerations to take into account when utilizing this data model.

Write i18n considerations.